Monster Bashes can occur at random times and also at pre-annouced scheduled dates. Dark Father has risen, time traveling dragons and medusa are types of monster bashes which often occur. Dark Fathers and Medusa will have 100% chance at dropping a rare. Time Traveling dragons monster bash is often a quick spur of the moment event in which you will get an increased amount of currency and small rares. Other more perpetual monster bashes such as the Infected Lord British Invasion will have multiple waves of monsters increasing in difficulty as the bash progresses, similar to a Champion Spawn.